you can get this crap called "root touch-up" which is hair dye designed specifically for the roots, maybe they use something similar to that cuz that can be used regularly with ease. But...Reita would have to have his hair bleached...so er...I dunno o.o Oh jeez he would look weird with his natural hair colour >.<
Bou from ancafe, his roots contrasted his dyed hair a lot, he even said he hated it.
I find Uruha's hair even more amazing, he keeps all the different colours in check right up into the roots...his hairstylist must hate him for having so much going on with it xDD
[edit] I dont study hairstyling I just pick things up...